Diagnosis on the bronchi of a subject is performed using a three-dimensional image data obtained by imaging the chest of the subject using a medical image diagnostic apparatus such as an X-ray CT apparatus. The diagnosis on the bronchi requires observation of temporal change of the bronchi in association with a breathing cycle. The cycle of breathing can be obtained by extracting a lung field region from three-dimensional image data and calculating the volume thereof.
However, the calculation of the volume of the lung field associated with the temporal change of the bronchi requires extraction of the lung field region from three-dimensional image data of a plurality of time phases in one cycle of breathing, which results in an increased time for processing image data.